1. Technical Field
The present invention relates to a control circuit and a control method of electromagnets, and more particularly, to a control circuit and a control method of elevator braking systems.
2. Description of Related Art
With the rapid development of electronic science and technology, the elevator technology has advanced rapidly as well. Specially, after being upgraded for several generations, the drive technology and control technology have evolved to permanent magnet synchronous speed regulation and microcomputer-based fully intelligent control respectively, which can enhance the reliability and stability of the complete machine. However, the brake control circuit (also referred to as contracting brake circuit), one of the major work circuits of elevators, is always designed in a conventional way.
In recent years, as the use of elevators increases sharply, the number of elevator induced safety accidents is on the increase as well. The brake fault induced accidents account for 80% of the total number of the accidents. In addition to mechanical faults, one major contributor to the brake fault is adhesion of contacts of switches of elevator braking systems, which leads to the inability of brakes to brake. The underlying cause leading to adhesion of contacts of switches is that the brake excitation coils are series connected in the contracting brake circuit thereby resulting in excessively large current flowing through the contacts of switches. Also, the continuous current of the brake excitation coil flows through the contacts of switches as well. As a result, the contracting brake circuit of the existing elevator braking systems cannot address the problem of such adhesion of the contacts of switches.
Substantially, the contracting brake circuit of the existing elevator braking systems falls into the categories as follows: 1. the contracting brake circuit that utilizes a current limiting resistance to achieve the switching between excitation voltage and holding voltage of the brake excitation coil; 2. the contracting brake circuit in which an arc quenching circuit is installed at the voltage switching contacts in order to increase the service life of the contacts; and 3. the contracting brake circuit that utilizes a rectifier diode to achieve the full-wave/half-wave rectification switching between the excitation voltage and holding voltage of the brake excitation coil.
In a typical contracting brake circuit as shown in FIG. 1, an operating contactor CJ, a door lock relay DJ, an economy resistance R, a contracting brake contactor ZJ and a brake excitation coil L are series connected after a full-wave rectification circuit D1-D4. A switch K is parallel connected across the economy resistance R, serving to achieve the switching between the excitation voltage and holding voltage.
In the contracting brake circuit, since the switching devices are connected in series with the brake excitation coil L, the excitation current flowing through the contracting brake circuit can normally be as high as several amperes. At the moment when the switch K is opened, the continuous current of the brake excitation coil L will flow through the diodes D3, D4 of the full-wave rectification circuit, which, along with the switch K, form a circuit. This will lead to arcing of the contacts of the switch.
In the full wave/half wave rectification voltage switching type contracting brake circuit as shown in FIG. 2, although the continuous current of the brake excitation coil L will not flow through the switch K, as the switch K is opened at any time randomly, when the switching occurs at the time when the current flowing through the brake excitation coil L reaches the maximum, the contacts of the switch K will be subject to the most severe arcing condition. In the event of adhesion of the contacts of the switch K, the brake will not be able to brake, causing the failure of the elevator braking system, and consequently, the major safety accidents such as the elevator slipping, overrunning or collapsing to the bottom.